The present invention relates to the fabrication of semiconductor devices. More specifically, the present invention relates to the fabrication of semiconductor devices, which use ultra high aspect ratio features.
Plasma-etching processes are commonly used in the fabrication of semiconductor devices. Generally, photoresist material forms feature patterns on the surface of the wafer to be etched, and features are then etched into the wafer by exposing the wafer to a particular type of etching gas. One of the challenges faced in plasma etching is the ever-increasing aspect ratio needed to meet design requirements, especially for ultra-high density structures. When etching features on semiconductor wafers, the aspect ratio of an etched feature is defined as the ratio between the feature's depth and the feature's critical dimension (CD). As more features are packed on a single piece of wafer to create higher density structures, the CD of each individual feature necessarily decreases, while the depth of the features remains unchanged. Thus, the aspect ratio of each individual feature increases as the device feature shrinks.
A new difficulty emerging recently during ultra-high aspect ratio (UHAR) etch is twisting, which is generally defined as deviations of location of the overlay. When the aspect ratio of a feature reaches a certain threshold while the feature's width is very small, twisting occurs, particularly near the bottom of the feature.